A man receives his blood panel results and one line stands out: vitamin D, 18 ng/mL. He spends twenty minutes looking up information on the web and the internet delivers what feels like a diagnosis: low vitamin D means low testosterone. Weak erections. Flat mood. Soft mornings. He buys a bottle that very same day and, as someone who does not operate at half-speed, goes with 5,000 units rather than the 1,000 unit version.
He checks his number six months later and the number is great. Everything else stays the same.
This is not a point about vitamin D being ineffective. Deficiency is real, it is widespread, and addressing it is worthwhile. This is an argument about the nature of laboratory numbers: a number may accurately represent your life without being a lever you can pull on it. The two are different objects, and much of the supplements industry is built around your lack of awareness of this distinction.
Sexual function in men demonstrates this distinction in its most pristine form. On the one hand there is an impressive collection of observational data: men with poor erectile function carry lower vitamin D, independent of the obvious explanations. On the other hand there is randomized controlled trial data, involving thousands of men being given vitamin D supplement for years. The two disagree, and why they do so is the most useful thing you will learn about your own physiology this month.
The association is real, and the supplement did not touch it
Let us start with the observational data, since it is genuinely solid. In late 2025 a systematic review of existing studies pooled 10 papers, covering more than 13,000 men. Men with moderate to severe erectile difficulty, especially those with vascular cause of said difficulties, have significantly lower serum vitamin D and worse performance on erectile function questionnaires. The association exists independent of lifestyle, cardiovascular risk factors, and sex hormones. Other papers placed some specific numbers on it: for each 10 ng/mL decrease in 25-hydroxyvitamin D the prevalence of erectile dysfunction increases by about 12%.
The mechanism is coherent, too. Erection is a vascular process above all else. It requires nitric oxide relaxing smooth muscles so that blood enters faster than it leaves, the exact process that punishes a man who grips harder when he realizes he is losing it. The active form of vitamin D participates in nitric oxide signaling and is important for the integrity of the endothelium, a single-cell layer that lines each and every blood vessel in your body. Lower vitamin D, worse endothelial function, worse erectile function. The sequence is logical.
So people did the logical thing. The D-Health Trial randomized 21,315 Australians, aged 60-84 to receive 60,000 IU vitamin D once a month or a placebo and measured erectile dysfunction three years after the start of the study. Vitamin D supplementation made no difference: not overall and not in subgroup and sensitivity analyses done after that.
Of course this trial has its weaknesses, and the 2025 review discusses them. It did not recruit men who were both deficient and struggling, which is the subgroup that is most likely to see a response. It used once-monthly dosing regimen, instead of daily dosing. Erectile function was assessed using one self-report question only. These are valid criticisms and the door is not shut completely.
But look at the concessions that the critics make. To salvage vitamin D as a treatment you need to narrow down to a deficient subgroup, change the dosing pattern, and measure more carefully. Quite far removed from the promise of the bottle, which is that raising the number raises the man.
The study everyone quotes, and the sentence nobody finished
The same shape can be observed in the testosterone literature, and here the myth is traceable to a single study.
In 2011, Pilz et al followed 54 men enrolled in a weight reduction program for a year. Of these 31 received 3,332 IU of vitamin D daily. For that group total testosterone increased from 10.7 to 13.4 nmol/L. Free and bioavailable testosterone increased along with total. No change in the placebo group. Every popular article about vitamin D and testosterone traces its roots to that table.
And now the part that never travels with that headline. The men were vitamin D deficient at the start of the study and had low testosterone. And as a later review of this literature puts it explicitly, no statistical analysis of the difference between groups was done. The famous result is a simple before/after analysis within one group, not a comparison of two groups to determine whether treatment beat placebo. It was a good signal to follow up on with randomized trials, which is precisely what the authors themselves asked for.
And the follow up trials arrived. In 2017, researchers from Graz gave 100 healthy men 20,000 IU weekly for 12 weeks, and found no effect on total testosterone. In 2019 the same team repeated the same protocol in 100 men with low testosterone levels, the population where the effect is expected to be the strongest. Again, no effect on total testosterone and no effect on any of secondary outcomes. Meta-analysis of 8 randomized trials found no effect on total testosterone and no effect on sex hormone binding globulin, regardless of dose, duration, dosing frequency, or baseline level.
The estrogen claim deserves its own sentence, because it circulates almost as widely. Estradiol was measured as a secondary outcome in both Graz studies and stayed unaffected. There is no randomized evidence that supplementing a man with vitamin D will decrease his estrogen.
Why loading up makes it worse
There is an instinct among high performing men that warrants a mention because it has the highest price tag. If a little is good, a lot delivered efficiently must be better. No need for daily nagging, pack the year in one go.
This particular instinct was put to the test. In 2010, Sanders et al gave 2,256 women older than 70 years old a single annual oral dose of 500,000 IU of vitamin D. Compared to placebo group, falls were increased by about fifteen percent and fractures by about twenty six percent. These adverse events occurred predominantly during the first three months following each yearly dose, which happened to coincide with peak concentration of the substance in blood.
The editorial comment accompanying the publication concluded quite sensibly that there is no proof of harm done from more frequent lower dose regimen and, therefore, daily, weekly or monthly doses are the logical way to go. Later research points towards some kind of counter-regulatory response, a physiological adjustment of the body against a flood. Indeed, a trial involving monthly regimens has seen more fallers in the two higher dose groups than in the lowest.
Read that in conjunction with your own habits. The dose regimen that felt most efficient proved to have the worst outcome, and the reason why is the same pattern that shows up when a man works his pelvic floor harder, or braces harder in bed. The system is not a ladder, where more input generates more output. It is a regulator, and it pushes back.
The threshold the experts withdrew
In 2011 the Endocrine Society issued guidelines that recommended 30 ng/mL as a target level. This figure was ubiquitous. It appeared on lab reports, in supplement marketing, and in the minds of men using it to measure themselves against. And in 2024 the same society issued new guidelines replacing the old ones. For healthy adults younger than 75 the recommendations advise against supplemental intake beyond the daily recommended allowances, and against routine testing of any kind, including in cases of adults with obesity and adults with darker skin color. Testing stays appropriate in case of interference with vitamin D metabolism by such things as kidney disease, malabsorption syndromes, granulomatous diseases, liver failure and certain medication use. As far as healthy adults are concerned, the panel could not define a level below which supplementation is necessary because the scientific literature did not provide it.
Four days before this briefing, the medical press pointed to the discrepancy between the guideline and the practice. Vitamin D continues to be one of the most ordered laboratory tests in American primary care and its volume did not change despite reversal of the recommendations. Nobody adjusted the order set.
None of the above means that vitamin D deficiency is invented out of thin air. It just means that the exactness you feel when looking at the result belongs to someone else. The threshold that made you worry was defined by a committee that later took it back.
What the sun is actually doing this month
Which leads us to the input responsible for generation of the major part of the number in the first place, and to the fact that right now it is late July.
The process is photobiological and not emotional. UVB radiation of wavelength roughly between 290 and 320 nm hits bare skin and transforms a cholesterol precursor into previtamin D3 that turns into vitamin D3. Any change in the amount of radiation reaching the skin affects the amount of vitamin D3 produced: season, time of day, day length, cloudiness, pollution, melanin content, use of sunscreen. Complete cloudiness halves the energy available; shade reduces it by about sixty percent. Older skin and darker skin produce less vitamin from the same exposure.
And there are two implications of this fact that stand out from others. First of all, there is a geographic one. Webb, Kline and Holick demonstrated decades ago that winter sunlight in Boston and Edmonton will not stimulate any significant vitamin D production in human skin at all. Above about the latitude of Memphis there are months in which no matter how much time a person spends outdoors, and no matter how disciplined he is, nothing will move the number.
And secondly, this is the implication that concerns men who think of themselves as outdoor types. UVB does not pass through glass. The sun shining from an office window, the car windscreen while commuting, the afternoon light in the living room, none of these produces even a drop of vitamin D. And a man can spend an entire summer in daylight and end up with the number he started with in February.
This is what the number was reporting all along. Not a lack of capsules. A life spent indoors, which very rarely comes alone. It comes with less activity, less sleep and more fat around the middle, and each of these pulls independently on the outcome a man blames on the vitamin.
Intel
Matukaitiene et al., 2025, ten studies and more than thirteen thousand men: lower vitamin D is associated with moderate to severe and vascular erectile dysfunction, irrespective of lifestyle, cardiovascular risk and sex hormones, based on low certainty observational evidence.
The D-Health Trial, 2024, 21,315 randomized participants, three years of 60,000 IU/month: no effect on the prevalence of erectile dysfunction, consistent across subgroups and sensitivity analysis.
Lerchbaum et al., 2017 and 2019, two randomized trials of 100 men each, one in healthy men and another in men with low testosterone: no effect on total testosterone and no effect on estradiol.
Sanders et al., 2010, 2,256 women aged 70 or older: an annual single 500,000 IU dose increased the risk of falls by about fifteen percent and fractures by about twenty six percent versus placebo.
The VITAL trial, 2019, 25,871 adults, 2,000 IU/day for a median of 5.3 years: no decrease in incidence of invasive cancer, major cardiovascular events or all cause mortality, and no increase in the risk of hypercalcaemia or any other adverse event. Cancer mortality showed a signal after exclusion of the earliest follow up years that has yet to be confirmed.
Order
There is no protocol in this briefing, because the honest account of this topic does not require one. What it requires is a shift in how you understand the number.
Stop interpreting the value as a verdict on your manhood and read it as a summary of the last six months of your behaviour. Think about what it is really answering. The number of hours spent on skin rather than on glass. The impact on sleep. The impact on movement. When a lab result and a way of living walk hand in hand, it is the latter that tends to move.
If you have symptoms that concern you, whether it is erectile dysfunction, persistent fatigue or anything that changed suddenly, the next step is a visit to a physician and not a purchase. Erectile dysfunction especially needs to be dealt with in a consultation room and not an aisle of supplements, because the vascular system reporting it is the same one driving the rest of you. And if you have a condition interfering with vitamin D metabolism, testing and treatment are clinical decisions that have a scientific foundation.
There is one final point worth making. Peak absorption is about thirty percent higher when the supplement is taken alongside a meal with fat content, and the same researcher found that meals affected absorption but not the resulting blood concentration in the subsequent weeks. That is the complete article in miniature. Men will optimize the timing of the capsule and leave the input that governs the whole number sitting untouched on the other side of a window.
The bottle might raise a value. Whether it changes anything else in you is another issue altogether. Fifteen years of trials have been answering it.